Static filter for long line data systems

ABSTRACT

A data system having data communication over a pair of electrical conductors includes a filter for eliminating static noise of atmospheric origin. In a specific embodiment, a system for acquiring seismic data includes a static filter for attenuating a common mode signal and atmospheric noise while preserving desired difference mode data. A specific embodiment of the filter includes a pair of input inductors having a piar of series connected capacitors and a pair of series connected resistors in parallel and connected across the conductors, with the nodes between the two capacitors and two resistors being connected to a common reference potential. The inductors are wound on the same core to effectively cancel the inductance presented to the difference mode signal.

Carroll 1 Dec. 11., 1973 STATIC FILTER FOR LONG LINE DATA SYSTEMS [75]Inventor: Paul E. Carroll, Houston, Tex.

[73] Assignee: Texas Instruments Incorporated,

Dallas, Tex.

1221 Filed: Dec.27, 1971 211 Appl. No.: 212,606

[52] 11.8. C1. 340/155 F, 333/12, 340/15.5 R [51] Int. Cl. 1104b 3/28,GOlv 1/22 [58] Field of Search 333/12; 340/155 F; 178/69 B [56]References Cited UNITED STATES PATENTS 2,280,950 4/1942 Harder 333/123,223,920 12/1965 Sasaki 333/12 2,733,412 1/1956 Alexander et al. 333/123,493,900 2/1970 Cushing 333/12 2,748,202 5/1956 McCallister et al.333/12 3,516,026 6/1970 Curran et a1 333/12 Primary Examiner-Benjamin A.Borchelt Assistant Examinerl'l. A. Birmiel Attorney-Harold Levine et al.

[57] ABSTRACT A data system having data communication over a pair ofelectrical conductors includes a filter for eliminating static noise ofatmospheric origin. In a specific embodiment, a system for acquiringseismic data includes a static filter for attenuating a common modesignal and atmospheric noise while preserving desired difference modedata. A specific embodiment of the filter includes a pair of inputinductors having a piar of series connected capacitors and a pair ofseries connected resistors in parallel and connected across theconductors, with the nodes between the two capacitors and two resistorsbeing connected to a common reference potential. The inductors are woundon the same core to effectively cancel the inductance presented to thedifference mode signal.

9 Claims, 5 Drawing Figures PAIENTED 1 I915 3.778.759

saw 1 a 2 v v STAT'C I ACQ EJ RS I TION .FILTER I I v SYSTEM.

SEISMIC Pmmmnu: i u ma SHEET 2 BF 2 OUT FREQUENCY (Hz) 1 STATIC FILTERFOR LONG LINE DATA SYSTEMS This invention relates generally toelimination of common mode atmospheric noise pickup in long line datasystems, and more particularly to such noise elimination in seismic dataacquisition systems.

According to a general method of seismic exploration, a seismicdisturbance is generated at or near the surface of the earth. Seismicwaves are thus generated and transmitted through the earth, and aredetected at various locations by. seismic transducers, commonly calledgeophones orseismometers. The electrical signal output of a group ofseismic transducers is typically coupled by a pair of electricalconductors, such as wire lines, to a centrally located seismic dataacquisition system for conversion to and storage in digital form,subsequently to be processed'in a digital data processing system.

The electrical signal output of a seismic transducer is a differencemode signal. The common mode signal, which is picked up by the wirelines from atmospheric disturbances, is the voltage potential common toboth wire lines. The difference mode signal is the difference voltagepotential between the two lines. The difference mode signal contains thedesired seismic data, but is of much less magnitude than the common modesignal. For example, thecommon signal my be a few volts whereas thedifference mode signal may be only a few microvolts. An input amplifierof the data acquisition system will somewhat suppress the common modesignal. Also, the signals may be transformer coupled into the dataacquisition system to provide additional low frequency common moderejection. However, effective filtering of atmospheric noise withoutattenuating the desired difference mode signal has been difficult toachieve.

In recentyears the practice has developed of placing seismic transducersat significant'distances from the data acquisition system, requiringconnecting lines of up to a mile and more in length. This practice hascompounded the noise problem in seismic data acquisition. The long wirelines tend to serve as antennas to pick up electrical static fromdistant atmospheric disturbances, such as thunder storms. The wire linesalso pick up static from near thunder storms and'from impact of windblown particles, such as sand and snow. Such noise pickup may degradethe signal to noise ratio of the signals on the'lines to such an extentas to render the data unusableJAs a result, many man and equipment hoursrequired for setting up a seismic shot may be lost, and may later haveto be duplicated at considerable expense.

Accordingly, the present invention comprises a filter, and a seismicdata acquisition system including such a filter, for attenuating staticnoise without significantly attenuating or otherwise impairing thedesired difference mode data.

It is a primary object of this invention to provide a filter forattenuatingsystem noise of atmospheric origin in long line data systems.

It is another object of the invention to provide a filter forattenuating a common mode signal and system noise of atmospheric originwhile preserving difference mode data in seismic data acquisitionsystems.

Additional objects and advantages of the invention will become apparentfrom the following detailed description in conjunction with thedrawings, in which:

FIG. 1' is a general block diagram of a seismic data acquisition systemincluding the filter of this invention;

FIG. 2 is a schematic diagram of a basic filter unit of this invention;

FIG. 3 is a schematic diagram of the single channel filter unit of FIG.2 illustrating coupling between the input inductors;

FIG. 4 is a plot of the frequency response of a specific embodiment ofthe filter of FIG. 3; and

FIG. 5 is a schematic diagram of a multi-channel filter. for a seismicdata acquisition system.

Referring now to the drawings, a general block diagram of a basicembodiment of this invention is illustrated in FIG. 1. The seismictransducer 1 detects seismic waves created by a seismic disturbance,e.g., an explosion of dynamite, and produces analog electrical signalsto be carried over wire lines 2 and 3 to data acquisition system 5. Indata acquisition system 5 the analog seismic data is converted todigital data and stored, subsequently to be processed in a dataprocessing sys tem. A static filter 4 is coupled between the seismictransducer 1 and data acquisition system 5. The static filter 4 filtersout static noise while preserving the detected seismic data. The seismictransducer 1 may be one of the various well known and commerciallyavailable types, for example, an Electro-Technical Labs Model EV20 orGeo Space Model GSC20-D. A suitable seismic data acquisition system 5 isdescribed in US. Pat. No. 3,134,957 by Robert S. Foote et al. andassigned to Texas Instruments Incorporated, as well as Models DFS-IIIand DFS-IV currently manufactured by Texas Instruments Incorporated.

Long data communication lines, such as lines 2 and 3 of FIG. 1, whichare exposed to the elements tend to pick up atmospheric disturbances andgenerate noise pulses, thus deteriorating the signal to noise ratio ofthe signals on the lines. Experimentation and analysis has led to theconclusion that the noise is of three basic forms, all having a broadspectrum including some very low frequency components. One source ofatmospheric noise is distant thunder storms, which are picked up like anantenna by the data lines. Noise from this source produces a dampedsinusoidal pulse or a differentiated pulse, dependent on line O. Thisnoise is particularly troublesome, since the thunder storms may be sofar away that the seismic crew 'will not know of their existence, yetthere may be sufficient pickup to substantially impart the seismic data.Noise pickup from more near thunder storms tends to produce adifferentiated pulse, also somewhat dependent on line Q. A third sourceof atmospheric noise is voltage buildup and resulting breakdown anddischarge resulting from impact with particulate matter, such as windblown sand or snow. Noise of this latter type may be eliminated byproviding a DC path from each wire to ground, but effective filtering isrequired to eliminate the static noise pickup from thunder storms.

The pulses created by thunderstorms may be several volts in amplitudeand have been determined to be predominately common mode in nature.Resulting problems are most significant at high gains when the usefuldifference mode signal is on the order of a few microvolts. The combinedcommon mode rejection of the desired filter and input amplifier of theseismic data acquisition system should be about db at 30 kHz for desiredperformance. Added resistance should not exceed 50 ohms per line andcapacity to ground should not exceed about 0.1 pf to prevent creation ofphase differences between the signals on the respective lines.

A schematic diagram of a static filter is illustrated in FIG. 2. Thecommon mode signal is carried on both lines 2 and 3, whereas thedifference mode signal is the difference potential therebetween. Thefilter suppresses the common mode signal and static noise whilepreserving the difference mode signal. The electrical signals enter thefilter through equal inductors 11 and 12. Series connected capacitors 13and 14 are connected across the two lines. Similarly, series connectedresistors l and 16 are connected across the two lines. The node betweenthe resistors 15 and 16 is connected to the node between the capacitors13 and 14, with the common node formed thereby connected to a referencepotential 17. In one specific embodiment wherein C,=C =0.l nf, L,=L 24 hand R,=R =5.1k().,

the filter provides an LR cutoff frequency of about 33 Hz and an LCcutoff frequency of about 320 Hz, and about 100 db of attenuation at 32kHz (neglecting distributed capacity in the inductors). The resistorshold the Q to less than 0.6 at resonance and provide a dc. path toground (or other reference). The d.c. path to ground for each wireeffectively eliminates the interference effects of sand, snow and otherwind blown particles. However, this filter also attenuates the desireddifference mode data as well as the common mode signal and noise.

The static filter of FIG. 3 is identical to the filter of FIG. 2, exceptfor the illustrated coupling between inductors 11 and 12. This may beachieved, for example, by winding both inductors 11 and 12 on the samecore. Thus the inductance presented to the common mode signals is thesame as for the filter of FIG. 2, but the inductors 11 and 12 of FIG. 3cancel each other as to the difference mode signal, thus eliminatingdifference mode filtering. This filter very effectively attenuates thecommon mode signal and static noise while leaving the difference modesignal unimpaired. The filter has the same frequency response as thefilter of FIG. 2, with the exception that only common mode signals areaffected. A plot of the frequency response is illustrated in FIG. 4,wherein the abscissa is common mode frequency and the ordinaterepresents attenuation in decibels. The filter frequency of FIG. 3 iseffectively a sharp cutoff common mode low pass filter. A practicaldisadvantage yet remains as to the large physical size of the inductors,which are 24 henrys nominal inductance in a specific embodiment.

A multi-channel filter for a seismic data acquisition system isschematically illustrated in FIG. 5. The multichannel filter iscomprised of a plurality of basic units, one for each channel, of thetype illustrated in FIG. 2. The nodes between the capacitor pairs C Cthrough C wherein there are] channels, are connected to common referencepotential 17. The input inductors of each filter channel are connectedby a pair of electrical conductors to a seismic transducer, and theoutput of each filter is connected by a pair of electrical conductors(typically quite short) to a particular channel of the data acquisitionsystem. All the inductors L L and L L may be wound on the same core toeffectively cancel the inductance presented to the difference modesignal, thereby eliminating difference mode filtering. Since there arenow 2j resistors connected in parallel, the effective resistance presented to each channel is R/2j, wherein R will be of the same nominalresistance (e.g., 5.1K) as the resistors of the filter of FIG. 3 for thesame filtering effect. Similarly, since there are now 2j capacitorsconnected in parallel, the effective capacitance presented to eachchannel is 2jC, wherein C is of the same nominal capacitance (e.g.,0.lp.F) as the capacitors of the filter of FIG. 3 for the same filteringeffect. This enables the inductors to be reduced in nominal value by afactor of 1/(2j) and yet retain the same LR and LC cutoffs. In a 12channel filter the required inductors would, e.g., be only 1 henry, ascompared to the '24 henry inductors required for the single channel unitof FIG. 3, for the same filtering effect.

The descriptionherein with reference to specific embodiments of theinvention is intended to be only illustrative of the principlesdisclosed.

What is claimed is:

1. In a system for acquiring seismic data of the type wherein at leastone seismic transducer is responsive to a seismic disturbance to producea difference mode electrical signal to be coupled by a pair of wirelines to a seismic data acquisition system, and wherein a common modesignal may be picked up by said lines from atmospheric disturbances,filter means coupled between said seismic transducer and said dataacquisition system for attenuating said common mode signal and noisewithout significantly attenuating said difference mode signal, whereinsaid filter means is comprised as follows:

a. a pair of magnetically coupled input inductors one connected to eachwire line;

b. following said inductors a pair of series connected capacitorsconnected across said wire lines;

c. a pair of series connected resistors connected across said wire linesin parallel with said pair of capacitors; and

d. means for connecting a node between said capacitors and a nodebetween said resistors to a common reference potential.

2. The system of claim 1 wherein said pair of inductors are wound on thesame core.

3. The system of claim 1 wherein said inductors are of equal inductance,said capacitors are of equal capacitance and said resistors are of equalresistance.

4. The system of claim 1 wherein said filter means is a multi-channelfilter comprised as follows:

a. a plurality of magnetically coupled inductors, one series connectedwith each wire line in said cable;

h. following said inductors, a pair of series connected capacitorsconnected across each pair of wire lines;

c. a pair of series connected resistors connected across each pair ofwire lines in parallel with said capacitors; and

d. means for connecting a node between each pair of capacitors and anode between each pair of resistors to a common reference potential.

5. The system of claim 4 wherein all of said inductors are wound on thesame core.

6. The system of claim 4 wherein all of said inductors are of equalinductance, all of said capacitors are of equal capacitance and all ofsaid resistors are of equal resistance.

7. A filter for use in attenuating common mode signals carried by pairsof electrical conductors in a multiconductor cable, comprising:

a. a plurality of magnetically coupled inductors, one

connected to each electrical conductor;

6 b. a plurality of pairs of series connected capacitors, capacitors anda node between each pair of resisone pair electrically connected acrosseach pair of tors to a common reference potential. electrical conductorssubsequent to said inductors; 8. The filter of claim 7 wherein saidinductors are of c. a plurality of pairs of series connected resistors,equal inductance, said capacitors are of equal capacione pairelectrically connected across each pair of 5 tance, and said resistorsare of equal resistance. electrical conductors and in parallel with apair of 9. The filter of claim 8 wherein all of said inductorscapacitors; and are wound on a common core.

d. means for connecting a node between each pair of

1. In a system for acquiring seismic data of the type wherein at least one seismic transducer is responsive to a seismic disturbance to produce a difference mode electrical signal to be coupled by a pair of wire lines to a seismic data acquisition system, and wherein a common mode signal may be picked up by said lines from atmospheric disturbances, filter means coupled between said seismic transducer and said data acquisition system for attenuating said common mode signal and noise without significantly attenuating said difference mode signal, wherein said filter means is comprised as follows: a. a pair of magnetically coupled input inductors one connected to each wire line; b. following said inductors a pair of series connected capacitors connected across said wire lines; c. a pair of series connected resistors connected across said wire lines in parallel with said pair of capacitors; and d. means for connecting a node between said capacitors and a node between said resistors to a common reference potential.
 2. The system of claim 1 wherein said pair of inductors are wound on the same core.
 3. The system of claim 1 wherein said inductors are of equal inductance, said capacitors are of equal capacitance and said resistors are of equal resistance.
 4. The system of claim 1 wherein said filter means is a multi-channel filter comprised as follows: a. a plurality of magnetically coupled inductors, one series connected with each wire line in said cable; b. following said inductors, a pair of series connected capacitors connected across each pair of wire lines; c. a pair of series connected resistors connected across each pair of wire lines in parallel with said capacitors; and d. means for connecting a node between each pair of capacitors and a node between each pair of resistors to a common reference potential.
 5. The system of claim 4 wherein all of said inductors are wound on the same core.
 6. The system of claim 4 wherein all of said inductors are of equal inductance, all of said capacitors are of equal capacitance and all of said resistors are of equal resistance.
 7. A filter for use in attenuating common mode signals carried by pairs of electrical conductors in a multi-conductor cable, comprising: a. a plurality of magnetically coupled inductors, one connected to each electrical conductor; b. a plurality of pairs of series connected capacitors, one pair electrically connected across each pair of electrical conductors subsequent to said inductors; c. a plurality of pairs of series connected resistors, one pair electrically connected across each pair of electrical conductors and in parallel with a pair of capacitors; and d. means for connecting a node between each pair of capacitors and a node between each pair of resistors to a common reference potential.
 8. The filter of claim 7 wherein said inductors are of eqUal inductance, said capacitors are of equal capacitance, and said resistors are of equal resistance.
 9. The filter of claim 8 wherein all of said inductors are wound on a common core. 